(1) Field of the Invention
The present invention relates to a hydraulic circuit structure for controlling the operating speed of a hydraulic actuator used on a construction machine or an agricultural or other working vehicle.
(2) Description of the Prior Art
A backhoe, which is one example of working vehicles, has a hydraulic motor, which is one example of hydraulic actuators, for driving a swivel deck. The operating speed of the hydraulic motor is controllable such that, the greater amount a manual swivel control lever is shifted from the neutral position to, say, a rightward swivel position, the faster the hydraulic motor rotates in a rightward swivel direction.
For controlling the operating speed of the hydraulic motor as above, the hydraulic circuit of the working vehicle includes an electromagnetically operable switch valve which is intermittently opened and closed under duty control, or an electromagnetic proportional reduction valve operable under voltage control. Thus, the greater amount the swivel control lever is shifted from the neutral position, the greater amount of oil is supplied to the hydraulic motor.
Such a control valve, generally, has three positions consisting of the rightward swivel position, neutral position and leftward swivel position. This control valve is urged to the neutral position, and includes solenoids for shifting the valve to the rightward and leftward swivel positions, respectively. A subtle difference in characteristics between the two solenoids results in a difference in the swiveling speed between rightward swiveling and leftward swiveling although the swivel control lever is shifted the same amount from the neutral position to either swivel position. Further, with this type of control valve, the flow rate control and directional control are effected simultaneously by a single mechanism. It is, therefore, impossible to effect half-port control and slow-speed control in a subtle and reliable manner.